Cytological studies in alloplasmic lines of maize
--L. Poggio*, C. A. Naranjo, C. L. M. Rosato* and L. B. Mazoti
*Also affiliated with Depto. Cs. Biologicas, FCEN, UBA
We studied the meiotic behavior of different alloplasmic lines of Zea mays ssp. mays, with cytoplasm of teosinte (Z. mays ssp. mexicana; Florida variety, Huixtla, Mexico). These lines were obtained by L. B. Mazoti using inbred lines of maize as the recurrent male parent, and teosinte as the cytoplasmic female donor. The maize inbred lines "c-tester", "gl ij" and "Multiple Dominant" were introduced in Argentina in 1933 by Ing. Agr. S. Horovitz, S, and they were maintained in the IFSC since their introduction. To obtain the alloplasmic lines, these inbred lines were backcrossed for 5 ("flint"), 7 ("c-tester", "gl ij" and "r-tester") and 20 ("Multiple Dominant") generations (Mazoti, 1978, 1987).
In the "Multiple Dominant" line with teosinte cytoplasm, Mazoti and Velazquez (1962) found a greater percentage of pollen sterility and greater variation of nucleolus diameter, than in the normal line. In addition, they found stickiness of meiotic chromosomes, and frequent intercellular contacts observed in sectioned anthers. Moreover, Mazoti (1987) reported that in this alloplasmic line, the knobs have greater size and higher DNA content. Poggio et al. (1990, 1991) using Feulgen microdensitometry, found higher DNA content per nucleus and higher heterochromatin percentage in the alloplasmic lines "Multiple Dominant" "c-tester" and "gl ij" than in the inbred lines. The same authors found that some individuals of the "Multiple Dominant" alloplasmic line showed desynapsis, cytomixis, cell fusion and pseudomultivalents in various places of the panicle.
In the present work we further analyze the meiotic behaviour and pollen
stainability of these alloplasmic lines. From the previous work as well
as from the present results we can list the following common features:
i) In the majority of the cells, the lines with teosinte cytoplasm showed 10 bivalents distributed in two groups of 5 bivalents each. This disposition was more remarkable than that found in the inbred lines (Fig. 1A, B).
ii) In about 20% of the PMCs in prophase, metaphase and anaphase I, the two groups of 5 bivalents are slightly asynchronous to each other (Fig. 1A).
iii) In contrast to the usual formation of only one nucleolus per PMC in Zea species and hybrids, two nucleoli were observed in 30% of the cells of the alloplasmic lines, each being associated with one of the groups of 5 bivalents (Fig. 1B). The presence of two nucleoli in the alloplasmic lines can be interpreted as if the teosinte cytoplasm permitted the expression of one inactivated NOR in the normal maize. There are evidences supporting a basic chromosome number of x=5 in the genus Zea (Molina and Naranjo, 1987; Poggio et al., 1990); therefore, each nucleolus could correspond to each genome of 5 bivalents, both being expressed in many of the cells of the alloplasmic lines.
These results support the hypothesis that maize is an allotetraploid and each group of 5 bivalents would correspond to a different genome. Additionally, the stainability of pollen grains was evaluated using the Alexander stain (1969). In the alloplasmic "Multiple Dominant" line with teosinte cytoplasm, a variation between 0-48% of stained pollen was recorded. This result agrees with that of Mazoti (1978) who found that this alloplasmic line shows mosaicism for pollen viability, and with that of Poggio et al. (1990), who found mosaicism for meiotic behaviour. In the other alloplasmic lines, pollen stainability was very low, 35% in r-tester and about 0% in the other lines in all anthers, and mosaicisms for meiotic behaviour were not observed.
1. A and B: see explanation in the text. Bars=10 um.
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